In power transmission networks alternating current (AC) power is converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion to DC power removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium, i.e. the transmission line or cable, and reduces the cost per kilometer of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance. A converter, such as a voltage source converter, provides the required conversion between AC power and DC power within the network.
A typical such power transmission network includes AC-DC converters, which are connected via a DC transmission link and are arranged to interconnect an AC electrical system and a DC electrical network.
U.S. 2010/085783 describes a method and a system to influence the power generation of at least one adjustable speed generator. The system includes a first voltage source converter connected to a local AC bus. The local AC bus is provided with power by the at least one adjustable speed generator. A second voltage source converter connected to an AC grid. A DC link is connected between the first and the second voltage source converter. At least one control unit controls the first and the second voltage source converters. The at least one control unit performs the method to control the AC voltage in the local AC bus via the first voltage source converter and to modify a reference value for the AC voltage magnitude of the local AC bus in dependence on the AC voltage magnitude of the AC grid.